Device for the measurement and monitoring of a process parameter

ABSTRACT

The invention concerns a device ( 1 ) for measuring and/or monitoring a process parameter, with a sensor ( 2 ), an intermittently working measurement circuit ( 3 ), which has at least one energy storer unit ( 16 ), wherein the measurement circuit ( 3 ) or individual components of the measurement circuit ( 3 ) are activated for a predetermined time span, the so-called active phase, and with a control center ( 14 ), wherein the measurement circuit ( 3 ) and the control center ( 14 ) are connected with one another over a two-wire line ( 12, 13 ) and wherein a control-/evaluation-unit ( 4 ) is provided, which activates the measurement circuit ( 3 ) at the earliest, when the energy in the energy storer unit ( 16 ) has reached a predetermined level.

[0001] The invention relates to a device for measuring and/or monitoringa process parameter. The device includes the following components: asensor; an intermittently working, measuring circuit, which has at leastone energy storage unit, wherein the measuring circuit, or individualcomponents of the measurement circuit, each are activated for apredetermined time span, the so-called active phase; and a controlcenter. The measurement circuit and the control center are connectedwith one another over a two-wire line. According to a preferredembodiment of the device, both the energy supply to the device and thedata exchange between the measurement circuit and the control centeroccur over the two-wire line.

[0002] Because of this double-function of the two-wire line and theassociated cost saving, two-wire measurement apparatuses are beingapplied to an increasing extent in industrial process technology. Animportant industry standard in this connection is the ISA-A50.1-Standard, in which direct current values between 4 mA and 20 mAcharacterize the particular measurement value and are transmitted overthe two-wire line.

[0003] Not quite problem free in the case of two-wire measurementapparatuses is that even in the case of a very small current level ofe.g. 4 mA there still must be enough power made available over thetwo-wire line to operate the measurement circuit, or its individualcomponents. The power supply issue here is naturally even more critical,the higher the power requirement of the measurement circuit, or itscomponents, becomes.

[0004] Basically, there are two possibilities for handling the problem.Either the measurement circuit is constructed of components with acorrespondingly small energy consumption—a solution, which enables acontinuous operation of the measurement circuit—, or components with arelatively high energy consumption are operated intermittently. In thecase of intermittent operation, energy is consumed only during theso-called active phase, while the recovery phase following thereafter isused for charging an energy storer, which then can supply the activecomponents of the measurement circuit with the required power again inthe next active phase. As an example of a two-wire measurement apparatusof the first named kind, U.S. Pat. No. 5,672,975 is cited. As an examplefor a device with intermittent operation, EP 0 687 375 B1 is named.

[0005] In the European patent, the measurement frequency of themeasurement value transmitter is so designed, that the correspondingpower demand is greater than the power available over the two-wire linein the case of minimum current and minimum voltage. Since the consumedpower exceeds the available power during the operation of themeasurement value transmitter, a deficit appears inevitably in the powerbalance. As soon as a sensing circuit recognizes a deficit, themeasurement circuit stops operation of the measurement program, untilthe deficit no longer exists. In short, in this known solution, adeficit is diagnosed in an energy storer. On the basis of this deficit,a longer cycle time is predicted to be necessary the measurementfrequency is then correspondingly changed. In the end, this means thatit is always estimated, when the energy storer will be completelycharged, or charged to a certain level. Following expiration of thisestimated time, a measurement signal is then issued. This known solutionhas the disadvantage, that it accepts excessively long inactive phases.Consequently, the measurement rate of the measurement system—and thusthe measurement accuracy of the fill level measurement apparatus—islowered.

[0006] The object of the invention is to propose a device for optimizingpower control in a fill level measurement apparatus.

[0007] The object is solved by providing a control-/evaluation-unit,which activates the measurement circuit at the earliest when the energyin the energy storer reaches a predetermined level. For this, the levelcan, for example, be sized such that it covers at least the energyrequirement of the measurement circuit during the active phase.Preferably, the level is, however, defined by the condition that theenergy storage unit is completely, or almost completely, charged.

[0008] Compared with the state of the art, the solution of the inventionexhibits three decisive advantages:

[0009] The device of the invention excels on the basis of an increasedreliability: While the measurement circuit of the prior art first reactswhen a critical voltage is exceeded, in the case of the solution of theinvention, the energy storer is always as completely charged aspossible. This is generally above all important, when DC/DC convertersare used between the energy storer and the energy consumer: In order todraw the necessary energy from the energy storer, the DC/DC converteraccording to I=U/R will take the current in inverse proportion to thestorer voltage from the energy storer. This means in the case of theknown solution that, for critically low storer voltages, the constantcurrent consumption from the energy storer increases, so that a recoveryof the system with decreasing voltage becomes always more improbable.

[0010] The device of the invention exhibits shorter cycle rates than thestate of the art: Since, in the case of an insufficient energy supply,the energy is held at a level near the maximum and not at a low,critical level, the voltage across the current regulator, which sets thecurrent, also decreases significantly. This residual voltage leads,moreover, as a matter of course, to a conversion of electrical energyinto heat according to the formula P=U·I. If the energy storer is alwayscharged completely, or almost completely, this voltage becomes lowerand, consequently, also the energy transformed into heat. The energyfraction gained hereby consequently additionally becomes available tothe components of the measurement circuit.

[0011] The device of the invention is parameter independent: Since thecycle time is not calculated, but, instead, the charge state of theenergy storer is watched, the device of the invention is independent ofthe constant current consumption, the pulse current consumption, and theenergy supply. It only must be guaranteed that the energy storer ischarged without pulse, which means that the current of the consumer mustbe smaller than the current supply, and the energy extraction during theactive phase must not drain the energy storer too far down. This is,however, in the end a question of the sizing of the measurement circuit.

[0012] According to a preferred further development of the device of theinvention, it is provided that the process parameter is the fill levelof a fill material in a container. In particular, the sensor is a filllevel sensor, which emits measurement signals in the direction of thesurface of the fill material and which receives echo signals reflectedat the surface of the fill material. The measurement signals areelectromagnetic signals, e.g. microwave signals, or ultrasonic signals.In the case of microwave signals, they either propagate in free field orthey are directed into the container over a conductive element.Corresponding measurement apparatuses are offered and distributed by theapplicant under the designations “Micropilot” or “Levelflex” or“Prosonic”.

[0013] The actual fill level measurement value is established using theso-called travel time method on the basis of the useful, or true, echosignal of the digital envelope curve, or the echo curve. Herein, theecho curve represents the amplitude values of the echo signals asfunction of the travel time of the measurement signal on the pathbetween the antenna and the surface of the fill material. Using the echocurve, the useful echo signal is determined, which represents the signalportion that is reflected at the surface of the fill material.

[0014] Devices that determine the fill level of a fill material in acontainer using the travel time of the measurement signal use thephysical law that the travel distance equals the product of travel timeand propagation velocity. In the case of fill level measurement, thetravel distance corresponds to twice the distance between the antennaand the surface of the fill material. The fill level then equals thedifference between the known distance of the antenna from the floor ofthe container and the distance of the surface of the fill material fromthe antenna, as determined by the travel time measurement.

[0015] If high-frequency microwave signals are used as the measurementsignals, then the echo signals are usually transformed into a lowerfrequency range using a sequential scanning, or sampling, method. Theintermediate frequency signal created by the transformation issubsequently evaluated. A characteristic of the intermediate frequencysignal is that it has the same course as the envelope; however, it isstretched relative to this by a defined time expansion factor. Theadvantage of the transformation to the intermediate frequency isprincipally that relatively slow and consequently cost-favorableelectronic components can be used for the signal registering and/orsignal evaluation. An embodiment of a method for sequential sampling ofecho-signals is described in DE 31 07 444 A1.

[0016] According to a preferred embodiment of the device of theinvention, the energy storer unit is a capacitor. A favorable furtherdevelopment, moreover, provides that a storer limiting unit is connectedin parallel with the capacitor. An example of a storer limiting unit isa Z-diode.

[0017] In order always to be able to be certain that the power needs ofthe fill level measuring apparatus are covered, an advantageousembodiment of the device of the invention provides that thecontrol-/evaluation-circuit initiates the active phase, as soon as apredetermined current value and/or voltage value has been reached in themeasuring circuit or in a component of the measuring circuit.

[0018] It has been found that it is particularly advantageous, when thecontrol-/evaluation-unit initiates the active phase first at that pointin time when the predetermined current value and/or voltage value is/areconstant over a predetermined time interval. In particular, thecontrol-/evaluation-unit provides an additional minimum wait time,before it initiates the active phase.

[0019] The invention is explained in greater detail on the basis of thefollowing drawings, which show as follows:

[0020]FIG. 1: a block diagram of one embodiment of the device of theinvention,

[0021]FIG. 2: a block diagram of the device of the invention,

[0022]FIG. 3: a flow diagram for initiating the active phase,

[0023]FIG. 4: a block diagram of a preferred embodiment of the clockedmeasurement circuit,

[0024]FIG. 5: a schematic drawing of the voltage transients, which areused preferably for establishing the clock rate of the measurementcircuit, and

[0025]FIG. 6: a drawing of different voltage curves on a storer unitthat is used in the invention.

[0026]FIG. 1 shows a block diagram of one embodiment of the device 1 ofthe invention. A fill material 11 is stored in the container 8. A filllevel measurement apparatus 1 serves for determining the fill level L.Apparatus 1 is mounted in an opening 10 in the lid 9 of the container 8.Transmission signals, especially microwave pulses, produced in thesignal production-/transmission-unit 5 are radiated from antenna 17 inthe direction of the surface of the fill material 11. The measurementsignals are reflected as so-called useful, or true, echo signals at thesurface of the fill material 11. These echo signals are received in thereceiver unit 6 and, if necessary, transformed to the intermediatefrequency. In particular, as already explained above, the time expandeddigital envelope curve is formed, which describes the amplitude valuesof the echo signals as a function of the travel time between antenna 17and the surface of the fill material 11. The correct clocking of thedeparture of the transmission signals and reception of the echo signalsby the transmission unit 5 and the reception unit 6 proceeds over thetransmission-reception separating filter 7. It is understood that,instead of the one antenna 17, separate transmission and receptionantennas can be used. According to the invention, the clock rate of themeasurement circuit 3 is so controlled by thecontrol-/evaluation-circuit 4, that it is assured that the powerrequirement of the device 1 of the invention is completely covered overthe two-wire line 12, 13 during the active phase, while the ‘firingrate’ is nevertheless as great as possible.

[0027]FIG. 2 is a block diagram of the essential components of thedevice 1 of the invention. The energy storer unit 16 is supplied withenergy through the energy supply unit 21. Preferably, the energy storerunit 16 is charged with a constant current. If the storer monitoringunit 23 recognizes that the predetermined level ‘PowerGood’ has beenreached in the energy storer unit 16, then the microprocessor 22receives the information that it can initiate the next active phase. Thelevel ‘PowerGood’ means, preferably, that the energy storer unit 16 isapproximately completely charged. Following that, the microprocessor 22triggers the transmission unit 5, and the transmission unit 5 issues ameasurement signal. The energy storer unit 16 supplies also themicroprocessor 22 with energy.

[0028] The initiation of an active phase, or the issuing of ameasurement signal, is preferably controlled according to a programstored in the microprocessor 22. PowerGood and MinCycleTime arepredetermined. As already mentioned, PowerGood characterizes preferablythe level defined by the energy storer unit 16 at approximately maximumcharge. MinCycleTime characterizes a maximum firing rate. This maximumfiring rate is not exceeded—not even when the available energy actuallywould be sufficient for an earlier introduction of the active phase.

[0029] According to the flow diagram shown in FIG. 3, when the level‘PowerGood’ has been reached, issue of the measurement signal, orintroduction of the next active phase, is delayed, until MinCycleTimehas expired.

[0030]FIG. 4 is a block diagram of a preferred energy supply of themeasurement circuit with current modulation. The energy storer unit 16is, in the illustrated case, a capacitor 18 with the capacity C_(S). Thecapacitor 18 is charged with a constant current Is until the voltageU_(S) reaches the maximum voltage of the Zener-diode 19 or, in general,the storer limiting unit. The diode 19 is selected such that theavailable energy has optimum characteristics for the components of themeasurement circuit 3 to be driven thereby. As soon as the voltage U_(S)has reached the maximum possible voltage D_(Z) on diode 19, the currentI_(S) stops flowing into the capacitor 18, but, instead, is turned intoheat by the Z-diode 20. As soon as this condition, and only when thiscondition, is completely reached or, theoretically, almost reached, theactive phase is initiated by the control-/evaluation-unit 4.

[0031]FIG. 5 shows schematically the voltage transients that arepreferably referenced for establishing the clock rate of the measurementcircuit 3. In particular, the typical course of the voltage U_(S) (seealso FIG. 4) versus time t is displayed in FIG. 5. The course of thevoltage is watched, for example, with a microprocessor, which is e.g.part of the control-evaluation-unit 4. During the active phase, thusduring the pulse duration T_(P), a measurement signal is issued in thedirection onto the fill material 11; the capacitor 18 is partiallydischarged. During the recovery period T_(R), the capacitor 18 is againcharged. During the minimum waiting time T_(F), the capacitor is notcharged further. In this time, the transient of the voltage runsparallel to the time axis. An observation of the voltage U_(S) forrecognizing of the horizontal transient during time T_(F) can, accordingto an advantageous embodiment of the device 1 of the invention, bereferenced as a criterion for when the measurement circuit 3 initiatesthe next active phase.

[0032] According to an alternative embodiment of the device 1 of theinvention, instead of the voltage transients, the voltage or current atthe storer limiting unit can be used as indicator for the initiating ofthe next active phase. For instance, if the current or the voltage iswatched at the diode 19, then a decision can be made as to whether theenergy storer unit 16 is completely charged.

[0033] Alternatively, the voltage across the current regulator can beused: If the supply voltage U_(B) is smaller than the voltage U_(S)resulting from the storer limiting unit (e.g. the voltage at the diode19), then the above-stated criterion is never reached. In this case, thevoltage across the current sink in fully charged condition will fallbelow a certain threshold. With reference to the example shown in FIG.2, this means that U_(B)−U_(S)=0V. Consequently, it is possible to usethe voltage across the current sink as another criterion for the nextactive phase. An optimum solution can be achieved by a combination ofthe two last-named criteria.

[0034] A further alternative for determining initiation of the activephase is provided by the calculation of the maximum voltage. If thelowest (after specification of the clamping voltage) voltageU_(S)=U_(TH) during the minimum waiting time T_(F) is calculated, thenthis can be used as a limit voltage for initiating the next activephase. Here, in particular, a simple comparator decides on theallowability of a new active phase. According to a preferred variant ofthe device 1 of the invention, it is additionally provided that thethreshold voltage U_(TH) can be a function of the current I. Thispermits further optimizing of the ‘firing rate’, or measurementfrequency, of the fill level measurement apparatus 1. In this case, theenergy storer unit 16 is no longer completely charged, since thetheoretical value for fulfilling the worst-case condition must besmaller than the actual voltage U_(S) during the minimum waiting timeT_(F).

[0035]FIG. 6 shows different voltage curves at an energy storer usedaccording to the invention for optimizing the firing rate of a filllevel measurement apparatus. An electrolytic capacitor 18 is used, forexample, as energy storer, wherein a main idea of the invention residesin holding this capacitor 18 always at a voltage level as close aspossible to the maximum available clamping voltage.

[0036] The behavior of voltage following a current consuming pulse—thusthe issuing of a measurement pulse—is shown in FIG. 6 for threedifferent starting levels. In this, the course of voltage of thecapacitor 18 is a measure for the energy level of the energy storer.

[0037] Preferred is that the capacitor 18 be in each case charged asclose as possible to the maximum clamping voltage U. The advantage ofthis solution according to the invention is, on the one hand, to be seenin that the loss power, which is consumed through the current regulator,is decreased; on the other hand, the energy level of the energy storeris regulated to a higher level. This is important above all in the caseof use of a DC-DC converter for power optimizing, since the currentwhich is drawn from the energy storer is inversely proportional to thevoltage. Consequently, also the recovery time is inversely proportionalto the relevant starting voltage—the higher the starting level, thesmaller the recovery time and the faster the following transmissionpulse can be issued.

[0038] Moreover, a voltage level, which approximates the maximumclamping voltage, lies far removed from the critical point. Seen ascritical voltage is that voltage level at which the voltage decrease asa result of the issuing of a measurement signal is so great that the‘point of no return’ is reached. When this voltage value is reached, thevoltage on the capacitor 18 falls to zero.

1. Device for measuring and/or monitoring a process parameter, with asensor (2), an intermittently working measurement circuit (3), which hasat least one energy storer unit (16), wherein the measurement circuit(3) or individual components of the measurement circuit (3) areactivated for a predetermined time span, the so-called active phase, andwith a control center (14), wherein the measurement circuit (3) and thecontrol center (14) are connected with one another over a two-wire line(12, 13) and wherein a control-/evaluation-unit (4) is provided, whichactivates the measurement circuit (3) at the earliest, when the energyin the energy storer unit (16) has reached a predetermined level. 2.Device as claimed in claim 1, wherein the process parameter concerns thefill level (L) of a fill material (11) in a container (8).
 3. Device asclaimed in claim 1 or 2, wherein the sensor concerns a fill levelsensor, which issues measurement signals in the direction of the surfaceof the fill material (11) and which receives the echo signals reflectedat the surface of the fill material (11).
 4. Device as claimed in claim1, 2 or 3, wherein the energy storer unit (16) concerns a capacitor(18).
 5. Device as claimed in claim 4, wherein a storer limiting unit(19) is connected in parallel with the capacitor (18).
 6. Device asclaimed in claim 1, 2, 3, 4 or 5, wherein thecontrol-/evaluation-circuit (4) initiates the active phase, as soon as apredetermined current value and/or voltage value is reached in themeasurement circuit (3) or at a component of the measurement circuit(3).
 7. Device as claimed in one or more of the preceding claims,wherein the control-/evaluation-unit (4) first initiates the activephase, when the predetermined current value and/or voltage value is/areconstant during a predetermined time interval.
 8. Device as claimed inone or more of the preceding claims, wherein thecontrol-/evaluation-unit (4) inserts an additional minimum wait time(MinCycleTime), before it initiates the active phase.
 9. Device asclaimed in claim 1, wherein both the energy supply of the device (1) andalso the data exchange between the measurement circuit (3) and thecontrol center (14) occur over the two-wire line (12, 13).